This invention relates generally to gas-insulated transmission lines, and more particularly to transmission lines having improved outer enclosures for containing the insulating gas.
Compressed gas-insulated transmission lines are a relatively recent commercial product which is gaining increased acceptance by the electric utility industry as an economical means for transmitting large blocks of electrical energy, and for use in special applications where conventional transmission lines are inadequate. A typical compressed gas-insulated transmission line is generally comprised of an elongated outer metallic sheath having disposed therein an inner conductor at high potential, with means disposed in the outer sheath to insulatably support the inner conductor within the outer sheath. An insulating gas, typical of which is sulfur hexafluoride at a pressure of 50 lbs/in.sup.2 gauge, is utilized for electrically insulating the inner conductor from the outer sheath. The inner conductor is typically used to transmit energy at voltage ranges from 121-1200 KV. Both the inner conductor and the outer sheath are of good electrically conducting materials such as aluminum.
One of the drawbacks of gas-insulated power transmission lines which is hindering even more widespread applications of the lines is associated with the cost of the lines. One of the more costly components of the transmission lines is the outer enclosure utilized to contain the inner conductor and the insulating gas.
Present gas-insulated transmission lines utilize a solid enclosure of aluminum to contain the inner conductor and the insulating gas. Aluminum is preferred because of the low electrical losses. However, the aluminum material utilized for the enclosure is itself expensive, and large quantities of the material must be utilized.
One method attempted in the prior art to minimize the cost of the outer enclosure has been to utilize carbon steel as the outer enclosure. Although carbon steel pipe is generally readily available at less expensive costs than aluminum, the losses which occur due to eddy currents are substantial. Therefore, because of the high losses occurring therein, the use of carbon steel for the outer enclosure has generally occurred only when the current rating of the line is very low and the high electrical losses are acceptable.
Another method attempted in the prior art is the use of an intercalated outer sheath, as specified, for example, in U.S. Pat. No. 4,095,040. This intercalated sheath utilized an insulating former which had wrapped around it overlapped layers of aluminum foil, both on the interior and exterior surfaces of the insulating casing.